Star Formation

The study of Star Formation addresses the question of how
stars are build up in galaxies and the early universe. Present
day star formation - which is the main focus of our research
group - takes place in molecular clouds and
giant molecular clouds (GMSs) like the Orion nebula. Within
these cloulds overdense cores might collapse under their own
weight until stars or brown dwarfs are formed. Observed stars
have a large range in masses (from a few hundred solar masses
down to the hydrogen burning limit of 0.07 solar masses). One of
the outstanding problems in star formation is the origin of
the Initial
Mass Function (IMF), which links the number of stars of a
certain mass range to their mass itself. The IMF can be
parametrised by a broken power law where the high mass range
(above one solar mass) follows closely a Salpeter type power
law (Salpeter 55). Large scale turbulent motions (which
is an active research topic by its own), for instance, could be
the driving force for the assembly of the spectrum of
self-gravitating cores which mass function (CMF) resembles the
IMF (e.g. Mac Low and Klessen, 2004).

Fragmentation

Many stars are accompanied by one or more companion
stars. These multiple star systems could be the result of
fragmentation during the collapse of turbulent cloud cores
(Dobbs, Bonnell and Clark, 2005) and/or the fragmentation of the
proto-stellar disk (Banerjee and Pudritz 2004). For
fragmentation to occur it is crucial that the collapsing cloud
core is able to cool efficiently. Therefore the chemical
composition and reactions influence strongly the fate of
collapsing objects. We are studying the evolution of condensed
cloud cores with numerical
simulations accounting for radiative processes and using
different equations of state (EOS).

Massive Stars

Unlike low mass, stars massive stars (> 8 solar masses ) face a
major obstacle during their assembly: Massive stars are still
accreting a large fraction of gas of their final mass while
they are already into their hydrogen buring phase. The
subsequently released radiation might be strong enough to halt
the accretion process and limit the final mass of the young
star. Spherical (Wofire and Cassinelli 1987) and two
dimensional models (Yorke and Sonnhalter 2002) showed that the
mass limit should be around 40 solar masses. On the other hand
observers find stars much more massive than this theoretical
predicted limit (e.g. Eta Carinae > 100 solar masses). We are
planing to address this problem with three dimensional numerical simulations which
will include the necessary radiative transfer to calculate the
radiation feedback.

Jets and Outflows

Observations indicate that young stellar objects (YSOs) are
accompanied by outflows and highly collimated fast moving
jets. These objects are referred to as Herbig-Haro
objects and are ubiquitously found in star forming regions. It
is still under debate how these outflows and jets are
launched. One possibility for the jet launching mechanism
could be the interplay of magnetic fields and the rotating
disk and/or proto-star. Numerical simulations including
magnetic fields during the collapse of cloud cores confirm
that outflows are generated during the star formation stage
(Banerjee and Pudritz 2006).